Forward link inter-generation soft handoff between 2G and 3G CDMA systems

ABSTRACT

In a CDMA cellular radiotelephone system, a soft handoff (SHO) is performed when a mobile station communicates with a new inter-generation base station, without interrupting communications with the old base station. Currently, a SHO can only be used between CDMA channels having identical frequency assignments and within the same system generation (i.e., 2G2G, or 3G3G, where 2G is a second generation system, and 3G is a third generation system). The proposed IS-2000 standard for a 2G3G handoff is a hard handoff or “Break-Before-Make” procedure, which greatly reduces the quality of service (QOS). The present invention allows for SHO between second and third generation CDMA systems (2G3G and 3G2G), by modifying the proposed messaging structure. This provides a smooth service transition when a mobile station travels from one service area (i.e., 2G), to another service area (i.e., 3G), using the SHO or “Make-Before-Break” approach.

This application claims priority from U.S. Provisional Application No.60/110,666, filed Dec. 2, 1998, entitled “Forward Link Inter-GenerationSoft Handoff Between 2G And 3G CDMA Systems”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to communication systems, andmore particularly, to a method for forward link inter-generation softhandoff between second generation (2G) and third generation (3G) CodeDivision Multiple Access (CDMA) systems.

2. Description of the Related Art

One commonly used type of cellular radiotelephone communication systemis referred to as a Code Division Multiple Access (CDMA) system. In aCDMA system, the radio signals share the same frequency spectrum at thesame time, in contrast to previous Frequency Division Multiple Access(FDMA) or Time Division Multiple Access (TDMA) systems. One current CDMAstandard, known as the second generation standard or 2G, is designatedas TIA/EIA-95-A/B (or IS-95-A/B), and is herein incorporated byreference. More recently, a new third generation (3G) CDMA standard hasbeen proposed and has been designated as IS-2000 (previously IS-95-C) orCDMA2000, and is herein incorporated by reference. As the new 3G systemsare installed, cellular systems will contain a mix of both old 2Gsystems and the new 3G compatible systems.

In a typical CDMA cellular radiotelephone communication system, a mobilestation communicates with a base station having the strongest availablesignal. In order to track the available signals, the mobile stationmaintains a list of available base stations. Specifically, each basestation in the CDMA system transmits an unmodulated “pilot” signal on aset of predetermined frequencies. A mobile station receives the pilotsignals and determines which pilot signals are the strongest. A“searcher” unit located in the mobile station commonly performs thesignal detection and strength measurement functions.

The results from the searcher are reported to the current (i.e. active)base station. The base station then instructs the mobile station toupdate a list of available base stations maintained by the mobilestation. The list is sub-divided into three operative sets—an activeset, a candidate set, and a neighbor set. The active set contains a listof the base stations with which the mobile station is currentlycommunicating (typically 1-4 base stations). The candidate set is a listof base stations which may move into the active set, and the neighborset is a list of base stations which are being monitored, but lessfrequently.

As the mobile station moves and its currently active base station signalweakens, the mobile station must access a new base station. Based uponthe results of the searcher, and the instructions received back from thebase station, the mobile station will update its sets, and communicatewith a different base station. In order for communication transmissionsto appear seamless to the user of the mobile station, the communicationlink must be handed off to the next base station. Ideally, this handoffwould establish a new link before terminating the first link. This typeof handoff is known as a soft handoff (SHO) or “Make-Before-Break.”

Presently, a SHO cannot occur between two different generations of CDMAsystems. The 3G system has been designed to provide backwardcompatibility with the 2G system at the signaling and call processinglevel. However, since these two systems employ different modulationschemes and spreading rates, they are not naturally compatible at thephysical layer. Therefore, at the service boundaries between the 2G and3G systems, a hard handoff, also known as a “Break-Before-Make” method,has been proposed.

In this type of hard handoff, the connection with a currently activebase station (i.e., 2G) is terminated before the new service with thenew base station (i.e., 3G) is established. This type of servicedisruption lowers the quality of service (QOS) for the cellulartelephone user. In this scenario, if the mobile station is engaged in avoice service, the user will most likely experience unpleasant voicequality degradation or even call drop. If the mobile station istransferring data, significant transmission delays (due toretransmission errors) will likely occur. In fact, the current standardcauses a minimum of 10 frames to be lost, before service is restored.

Thus, it would be desirable to provide a soft handoff between twodifferent generations of CDMA systems, in order to avoid thedisadvantages associated with the currently proposed hard handoffscheme.

SUMMARY OF THE INVENTION

The present invention is a modification to the proposed IS-2000specification, in order to provide soft handoffs on forward linksbetween two different generations of CDMA systems. In general, thepresent invention modifies the proposed messaging structure to allow forreporting of the generation type of the base stations. Two differentembodiments are disclosed, as well as two possible soft handoffprocedures. The present invention is not limited to the disclosedpreferred embodiments, however, as those skilled in the art can readilyadapt the teachings of the present invention to create other embodimentsand applications.

In a first embodiment, a system configuration parameter is added to theGeneral Handoff Direction Message, and Extended Handoff DirectionMessage. The Neighbor List Message and Extended Neighbor List Messageare updated to include information concerning both systems' basestations. A selection based soft handoff is used to perform the handoffbetween inter-generation systems. The selection is based on the signalstrength of the received pilot signals.

In a second embodiment, four parameters are added to the PILOT_PN recordof the General Handoff Direction Message. The four parameters are ageneration identification parameter, a radio configuration parameter, adrop timer parameter and a drop threshold parameter. Based upon thevalues of the generation identification parameter and the radioconfiguration parameter, either a selection based soft handoff or a truehandoff is performed. The true handoff combines signals from bothgeneration systems, before dropping a current base station in favor ofthe stronger other generation base station. The drop parameters may beused to provide a sufficient time overlap for the two different basestations, and to allow a system designer to tune the network.

BRIEF DESCRIPTION OF THE DRAWINGS

The exact nature of this invention, as well as its objects andadvantages, will become readily apparent from consideration of thefollowing specification as illustrated in the accompanying drawings, inwhich like reference numerals designate like parts throughout thefigures thereof, and wherein:

FIG. 1 is diagram illustrating one possible IS-2000 deployment scenario;

FIGS. 2 and 3 are tables illustrating a Neighbor List Message;

FIG. 4 is a table illustrating one embodiment of the modified NeighborConfiguration Table;

FIG. 5 is table showing an embodiment of the modified PILOT_PN record;

FIG. 6 a table illustrating the permissible inter-generation softhandoffs according to one embodiment of the present invention;

FIG. 7 is a table illustrating the additional parameters added to thePILOT_PN record of the General Handoff Direction Message according to apreferred embodiment of the present invention;

FIG. 8 is a table of the simulation parameter settings used for an AWGNcomputer simulation;

FIG. 9 is a table of the simulation parameter settings used for a fadingcomputer simulation;

FIG. 10 is a graph of the results of the AWGN computer simulation;

FIG. 11 is a graph of the results of the fading computer simulation; and

FIG. 12 is a block diagram of a CDMA system configured to operateaccording to the present invention.

DETAILED DESCRIPTION

The following description is provided to enable any person skilled inthe art to make and use the invention and sets forth the best modescontemplated by the inventor for carrying out the invention. Variousmodifications, however, will remain readily apparent to those skilled inthe art.

The current second generation (2G) CDMA systems, designated asTIA/EIA-95-A/B (or IS-95-A/B) systems, are being upgraded and willultimately be replaced by the third generation (3G) CDMA systems. Theair interface of the 3G (IS-2000) system uses a new modulation scheme toallow better spectral efficiency, as well as different spreadingfactors. However, a part of the new 3G system, which operates within thesame channel bandwidth as the old 2G system, is required to becompatible with the 2G system at the signaling and call processinglevel. The reverse link of the 3G system, though, employs coherentdemodulation, whereas the reverse link of the 2G system employsnon-coherent demodulation. Thus, in the 3G specification, there was noattempt to make these two systems compatible at the physical layer.

Additionally, the forward links of the two systems use differentmodulation methods (QPSK (3G) vs. BPSK (2G)), which require somemodifications within the new 3G system's demodulator. However, since theIS-2000 terminal (i.e. mobile station) must be able to operate in theIS-95-A/B network, the new 3G terminal is required to be able to switchits mode of operation from one system to the other automatically.

In practice, it is impractical to perform a reverse link SHO between 2Gand 3G systems because the 3G base station cannot demodulate a 2Greverse link and vice versa (coherent vs. non-coherent, differentmodulations, etc.). However, according to the present invention, amethod for performing a SHO on the forward link is disclosed that can beimplemented with only a few minor modifications to the proposed 3Gsystems.

A mobile station receiver comprises a “rake” receiver and several othercomponents. The rake receiver consists of several (at least three fornarrow band CDMA), demodulating elements (or “fingers”). These multipledemodulating elements or fingers function like a garden rake to “rake”in the signals, thus the name. Each of these demodulating elements iscapable of independently tracking and demodulating a multipath componentreceived from a single base station or a signal from several basestations (up to the number of demodulating elements in the rakereceiver). Therefore, it is possible to configure one demodulatingelement according to the 2G standard and at least one other demodulatingelement to the 3G standard. Thus, a single mobile station can be usedacross a mixed generation system.

In one preferred embodiment, the deployment model of the 3G systemconsists of a partial overlay of the current 2G (IS-95-A/B) networks10,12 and the new 3G (IS-2000) network 14, as shown in FIG. 1. As themobile station travels from one generation 121 to another generation 141of the network, according to the proposed hard handoff, the station willbe forced to drop the current service before it re-establishes theservice on the other generation network. According to the presentinvention, a few modifications to the standard proposed message formatsin the IS-2000 specification are made, in order to provide a SHO for theforward link. Specifically, changes to the messaging structure areproposed to allow for the reporting of 3G base stations. The presentinvention thus allows for a soft handoff between two generations ofsystems, which allows for maintaining the QOS across the generationboundaries.

The present invention creates a mechanism for informing the mobilestation about the surrounding network, such as the network parameters(data rates, etc.) and whether the network is a 2G, 3G, or 2G/3G mix.This can be achieved by an addition of a 1-bit field into the GeneralHandoff Direction Message, and Extended Handoff Direction Message.Additionally, each base station in the 2G/3G service overlap area musthave all the other system's base stations stored into the Neighbor ListMessage and Extended Neighbor List Message. This can be achieved byadding a new definition to the NGHBR_CONFIG field.

Preferably in the network deployment, the same base station controllerwill supervise the two different generations of base stations.Therefore, for the generation overlay area, the Neighbor List Messageand the Extended Neighbor List Message include both types of systems.FIGS. 2 and 3 illustrate the definition of the Neighbor List Message20,30. The Neighbor Configuration Table 40 is then modified as shown inFIG. 4, wherein the underlined entries are examples of the proposedmodifications. Specifically, two new channel configuration entries 401,402 have been added (one for the 2G and one for the 3G system).

After receiving the Neighbor List Message or the Extended Neighbor ListMessage, the mobile station measures the pilot signals in the active,candidate and neighbor sets and reports the strengths to the basestation, using the Pilot Strength Measurement Message. This procedure isperformed by the searcher, which computes the strength of each pilot byadding the ratio of the received pilot energy per chip, Ec, to the totalreceived spectral density (noise and signal), lo.

Since the base station knows which of the detected and reported pilotsare associated with second or third generation system, the base stationcan use this information to manage the forward link SHO capability.Specifically, compatible base stations are added to the mobile station'sactive set. This is done through the General Handoff Direction Message(GHDM) and Extended Handoff Direction Message (EHDM). A field indicatingthe generation (2G, 3G) 51 of the associated PILOT_PN record 50 is alsoadded. The PILOT_PN record 50 of the message can be modified, forexample, as illustrated in FIG. 5. The underlined field“2G/3G_CHAN_CONFIG” 51 has been added.

Before the GHDM message is sent to the mobile station indicating thatthe forward link SHO between the 2G and 3G systems should be performed,the base station controller allocates the necessary channel resourcesfor each of these two systems (since, as noted above, the samecontroller supervises both generations).

Since each mobile station contains at least three demodulating elements(fingers) in its rake receiver, it may assign one of these fingers to ademodulated signal arriving from the 2G base station, while theremaining fingers demodulate the signal arriving from the 3G basestation (or vice-versa). According to the proposed specification, eachIS-2000 mobile station must be capable of demodulating an IS-95-Bsignal, therefore it is possible to demodulate these two signalsindependently (note that in an overlay deployment the two modulationsignals will be orthogonal to each other). Additionally, since thecomputational requirements (channel decoding) and the interleaver memoryof IS-2000 mobile stations are large (to sustain the maximum datarates), the capability to demodulate and decode these two independentchannel configurations are already within the capability of thecurrently defined mobile stations.

According to this embodiment of the present invention, the SHO proceduremay be performed as follows:

1. If the mobile station is in the 2G/3G overlay area, the base stationincludes pilots belonging to both systems into the Neighbor List Message(NLM).

2. The mobile station measures the pilot strength of all base stations(2G and 3G) and reports them to the base station.

3. If the 2G (or 3G) pilot Ec/lo>T_ADD threshold, the base stationincludes this pilot into the mobile station's active set.

4. The mobile station continuously demodulates the current generationbase station assignment.

5. The mobile station assigns one or more demodulating fingers to the“other generation” base station signal, and it demodulates and decodesthe information independently from the current assignment.

6. After decoding the first good frame from the “other generation” basestation, the mobile station starts a Tm counter, and upon itsexpiration, reports the event in Handoff Completion Message (the Tmcounter must be defined, as is used to determine the SHO timing).

7. The base station may now drop the “other generation” pilots(channels) from the mobile station's active set, thus completing theSHO.

The above described embodiment is a selection based SHO, orinter-generation selection based SHO (ISBSHO), that is, in an overlayregion the mobile station receives two base station signals (one fromeach generation) and decides which signal is stronger. The base stationwith the strongest signal is then selected and the weaker signal isdropped as described above.

A second type of SHO is referred to herein as a “true” SHO. As definedherein, a true SHO occurs when the two signals from the two differentgeneration systems are actually combined together, before one signal isdropped. Currently, when a mobile station is communicating with a basestation, a finger is assigned to each multi-path component. The signalsare then combined together before the bit is decoded. The true SHO ofthe present invention proposes assigning a least one finger to the“other” generation signal, such that after the signals are demodulatedand interleaved, the soft symbols are combined and decoded to produce anoutput bit. Thus, in contrast to the first embodiment of the presentinvention, the mobile station is actively using two signals from twodifferent inter-generation base stations simultaneously, in an overlayregion. Once one signal becomes too weak (i.e. the signal strength dropsbelow a threshold), that signal is dropped and the mobile stationcommunicates only with the stronger base station.

A true SHO approach, however, can only be used if the coding rates ofthe two different signals are the same. If the coding rates aredifferent, the signals must be decoded sequentially, and the selectionbased SHO must be used. Thus, in mixed generation and signalenvironment, sometimes a selection based SHO scheme is required. When atrue SHO can be performed, though, it is preferred. A second embodimentof the present invention will now be described which implements a trueSHO, when possible.

The table shown in FIG. 6 specifies when each type of inter-generationsoft handoff is permitted. As seen in the table, the 2G systems supporttwo data rates (RS-1 and RS-2), while the 3G systems may have five ormore data rates. As illustrated, in addition to the SHO between RS-1RC-1and RS-2RC-2, a true SHO (SHO) can be performed when the coding and thedata rates transmitted on both forward links are the same (i.e.RS-1RC-4). The inter-generation selection based soft handoff (ISBSHO)can be performed when both forward link data rates are equal but thecoding rates are different (i.e. RS-1RC-3 and RS-2RC-5). As newconfigurations are added, the present invention may be applied asdescribed herein, based upon the data and coding rates.

When performing a true SHO, the mobile station assigns one or more ofits fingers to demodulate the IS-95A/B base station signal and theremaining fingers to the IS-2000 base station. The received signal isdemodulated according to the modulation and spreading parameters of therespective base station, and the demodulated symbols are combined in themaximum ratio (MR) fashion before decoding, similar to the normal SHO.For the 2700 bps and 1500 bps rates of RC-4, after demodulation, thesymbols are de-punctured and then only the information symbols (not theCRC symbols) are combined. When performing an ISBSHO, the receivedsignal is demodulated according to the modulation and spreadingparameters of the IS-95A/B and IS-2000 base stations. Signal componentsfrom the same base station are added in an MR combiner and then decodedsequentially by the decoder. The best frame can be then selected basedon the frame quality.

Since an IS-2000 mobile station is capable of receiving and demodulatingboth an IS-95A/B and an IS-2000 signal, a simple extension to themessage structure will allow the simultaneous demodulation of signalsfrom both generations permitting soft handoffs. This can be achieved byadding four new fields to the PILOT_PN record 70 of the General HandoffDirection Message. The following four new parameters are added to thePILOT_PN record 70 of the General Handoff Direction Message: ageneration identification parameter (IS-95B_IS-2000) 71, a radioconfiguration parameter (RADIO_CONFIG) 72, an inter-generation droptimer parameter (IG_T_DROP) 73, and an inter-generation drop threshold(IG_DROP_TSHD) 74. These additions are illustrated in the table of FIG.7.

The IS-95B_IS-2000 field 71 is used to identify the generation type (2Gor 3G) of a base station. The RADIO_CONFIG field 72 specifies the datarate, spreading rate and code rate (i.e. all the modulation parameters).For example, if the IS-95B_IS-2000 field 71 is a “0” (2G), then only 1bit is used for the RADIO_CONFIG field 72 to specify the data rate—“0”for RS-1, and “1” for RS-2. If the IS-95B_IS-2000 field 71 is a “1”(3G), the RADIO_CONFIG field 72 defines which configuration isapplicable (RC-1-RC-5). Additional 3G configurations are envisioned, soin the preferred embodiment the RADIO_CONFIG field 72 has been shown as4 bits, however fewer bits may be used, or additional bits added toprovide for even more configurations.

The IG_T_DROP field 73 is a timer used to determine the length of theSHO. For example, the IG_T_DROP timer 73 may be implemented dynamically,so that each base station specifies its value independently. This wouldallow the network to be “tuned” dynamically. In a preferred embodiment,the range of values of the IG_T_DROP timer 73 is 0-15 frames. Finally,the IG_DROP_TSHD field 74 is a threshold value that uses an energymeasure to determine when to drop a base station. Specifically, theIG_DROP_TSHD field 74 uses the Ec/lo energy measure to determine when asignal is too weak to be of any use, and is therefore dropped. In apreferred embodiment, the IG_T_DROP timer 73 is used for the selectionbased SHO, while the IG_DROP_TSHD threshold 74 is used for the true SHO.However, the two measures may also be used together to further refinethe SHO mechanism. Once one signal is dropped, the mobile station istuned to use only the other base station.

According to a preferred embodiment of the present invention, the SHOprocedure may be performed as follows:

When a mobile station is in the IS-95A/B-IS-2000 overlay area, pilotsbelonging to both generations are included in the Neighbor List Messages(NLM) or Neighbor List Update Message (NLUM).

The mobile station measures the pilot strength of all base stations (2Gand 3G) and reports their strength to the active base station in thePilot Strength Measurement Message (PSMM).

If the inter-generation candidate pilot Ec/lo reported in the PSMMmessage is larger than the T_ADD threshold, the base station includesthis pilot in the GHDM message, indicating system generation, radioconfiguration and handoff parameters.

The mobile station then assigns one or more demodulating fingers to the“other-generation” base station signal, and demodulates and decodes theinformation depending on the radio configuration and handoff parameters.

When all the inter-generation handoff requirements specified in the GHDMmessage (IG_T_DROP and/or IG_DROP_TSHD) are satisfied, the mobilestation terminates its transmission on the “current” generation link(using the SHO permitted as shown in FIG. 6) and starts transmitting onthe “other” generation link.

The mobile station completes the inter-generation handoff by sending theHandoff Completion Message.

Computer simulations of the true SHO are shown in FIGS. 10 and 11.Simulations were run in an AWGN and a fading environment. For the AWGNcase, the simulation parameters were set as shown in FIG. 8. The RS-1curve shows the FER when just the single path from the IS-95B basestation is used. The RS-1RC-4 curve shows the gain when one path fromeach base station is used in a true inter-generation SHO scenario. Thesimulation parameters for the fading case are shown in FIG. 9, with themobile station speed set at 30 km/hr. For the fading environment, theRS-1 curve shows the FER when just the single path from the IS-95B basestation is used. The RS-1RC-4 curve shows the gain when one path fromeach base station is used in the true inter-generation SHO scenario. TheISBSHO performance was not simulated since the performance gains arewell understood by those skilled in the art and depend only on the powerof the link from each of the two different generation base stations.

As described, the present invention provides a simple mechanism tofacilitate inter-generation soft handoffs on a forward traffic channelin a mixed-generation CDMA cellular radiotelephone system. The inclusionof a true inter-generation soft handoff into the IS-2000 standard cansignificantly simplify the deployment of 3G systems, since there is noneed to include IS-95B channel elements in the IS-2000 base stations.Also, the soft handoff overcomes the shortcomings of the proposed hardhandoff procedure, without increasing the system complexity or hardwarerequirements. No additional complexity is added to the mobile stationssince each IS-2000-1x mobile station must already be capable ofdemodulating an IS-95-B signal, and it can demodulate two differentgeneration signals independently.

An example of a CDMA system 120 incorporating the present invention isshown in FIG. 12. A mobile station 124 communicates with a first basestation 122. As the mobile station moves, it must be handed off to acloser base station 123. As new 3G systems are introduced, a CDMA system120 will have a mixture of both 2G and 3G systems. According to thepresent invention, a common base station controller 121 controls boththe 2G and 3G base stations 122, 123. In this case, for example, thefirst base station 122 may be a 2G system and the second base station123 may be a 3G system.

If the mobile station 124 and the second base station 123 are configuredaccording to the present invention, the mobile station 124 makes aforward link with the second base station 123, before terminating thelink with the first base station 122. This “soft handoff” improves theQOS for the mobile station, as compared to a hard handoff.Interestingly, this improvement can be accomplished without significantadditional hardware complexity.

Those skilled in the art will appreciate that various adaptations andmodifications of the just-described preferred embodiments can beconfigured without departing from the scope and spirit of the invention.For example, additional messages may be added or the data structuresmodified in the proposed IS-2000 specification to produce the sameresults as those described herein. Furthermore, the present inventionmay be extended to the European CDMA implementations, to allow handoffbetween GSM and W-CDMA. Therefore, it is to be understood that, withinthe scope of the appended claims, the invention may be practiced otherthan as specifically described herein.

What is claimed is:
 1. A method to be performed by a mobile device, themethod comprising: receiving a first data signal from a first systemthat uses a first modulation scheme; receiving a second data signal froma second system that uses a second modulation scheme, the secondmodulation scheme being different from the first modulation scheme;receiving from a system controller modulation information identifyingthe modulation scheme used by the second system; performing a softhandoff between the first and second system by: using the receivedmodulation information to select a soft handoff procedure from aplurality of soft handoff procedures including a first procedure and asecond procedure; if the first procedure is selected: demodulating anddecoding the first data signal while independently demodulating anddecoding the second data signal; and dropping the first data signal upondetermining that the second data signal is stronger than the first datasignal; if the second procedure is selected: demodulating the first datasignal while independently demodulating the second data signal;combining the first and second data signals and decoding the two signalstogether; and dropping the first data signal upon determining that thesecond data signal is stronger than the first data signal.
 2. The methodof claim 1, wherein: the first system is an IS-95 CDMA system and thesecond system is an IS-2000 CDMA system.
 3. The method of claim 1,wherein: the first and second modulation schemes differ in that theyhave different spreading rates.
 4. The method of claim 1, wherein: eachmodulation scheme has a spreading rate, a data rate and a coding rate;and the received modulation information identifies the spreading, data,and coding rates of the second modulation scheme.
 5. The method of claim4, wherein using the received modulation information to select a handoffprocedure includes: using the received modulation information todetermine whether the first and second modulation schemes have the samespreading, data, and coding rates.
 6. The method of claim 5, wherein:the first procedure is selected if the first and second modulationschemes have the same data rates but different spreading and codingrates.
 7. The method of claim 5, wherein: the second procedure isselected if the first and second modulation schemes have the same dataand coding rates, but different spreading rates.
 8. The method of claim1, wherein determining that the second data signal is stronger than thefirst data signal includes: determining that the strength of the firstdata signal has dropped below a threshold energy value.
 9. The method ofclaim 8, wherein: the received modulation information specifies thethreshold energy value.
 10. The method of claim 1, wherein determiningthat the second data signal is stronger than the first data signalincludes: determining that the number of good frames that have beendecoded from the second data signal meets a threshold number of frames.11. The method of claim 10, wherein: the received modulation informationspecifies the threshold frame number of frames.